Yet another one-liner. This is a bit tricky, and I can't say I'm particularly proud of it. It assumes  $startdir always has a trailing  \ (backslash) delimiter. Needs Perl version 5.14+ for the  s/// substitution  /r modifier.

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.014;
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my $plsname =
   $curdir =~ s{ \A (?i) \Q$startdir\E | \\ }{ $-[0] ? '_' : '' }xmsre
+g . '.pls';
 print qq{'$plsname'};
"
'Schubert_Lieder_Terfel.pls'
[download]

But again, I must say I can't see anything really wrong with the basic approach of the OPed code.

Update: Even more perverse, but requires no  /r modifier, so it can run under pre-5.14, and no  /e modifier:

c:\@Work\Perl\monks>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my %replace = ('1', '',  '-1', '_',  '0', '.pls');
 (my $plsname = $curdir) =~
   s{ \A (?i) \Q$startdir\E | \\ | \z }
    {$replace{ $-[0] == 0 || $-[0] <=> $+[0] }}xmsg;
 print qq{'$plsname'};
"
perl version 5.008009
'Schubert_Lieder_Terfel.pls'
[download]

The Devil (but not Terfel) finds work for idle hands.

Update 2: Ok, slightly simpler, but needs captures:

c:\@Work\Perl\monks>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $curdir   = 'Y:\Music\Schubert\Lieder\Terfel';
 my $startdir = 'Y:\mUsIc\\';
 ;;
 my %replace = ('1', '',  '2', '_',  '3', '.pls');
 (my $plsname = $curdir) =~
   s{ (\A (?i) \Q$startdir\E) | (\\) | (\z) }
    {$replace{ $#- }}xmsg;
 print qq{'$plsname'};
"
perl version 5.008009
'Schubert_Lieder_Terfel.pls'
[download]

Good luck maintaining this.